1. Field of the Invention
The invention relates to an engine control system for an engine for controlling the amount of fuel injection according to a target air-fuel ratio determined on the basis of a target engine load.
2. Description of the Related Art
Typically, in an engine performing stratified charge combustion to operate with a lean fuel mixture for which an air-to-fuel ratio is controlled according to according to engine operating conditions, control of the amount of fuel injection is made based on target air-to-fuel ratio determined based on a target engine load or a target value of a factor corresponding to an engine load according to engine operating conditions. A control system for a direct injection-spark ignition type of engine described in, for example, Japanese Unexamined Patent Publication No. 7-301139 determines target engine output torque meeting a target engine load on the basis of an engine speed and an throttle opening and further determines a target air-fuel ratio according to the target output torque by an operation with a map. The amount of fuel injection meeting the engine demand is calculated by using the target air-fuel ratio and the amount of intake air. The amount of intake air is controlled by means of a motor driven throttle valve whose opening is controlled according to accelerator pedal travel.
If simply controlling the amount of fuel injection on the basis of a target air-fuel ratio according to a target engine load or output torque only likely by the engine control system described in the above mentioned publication, the engine does not always produce output torque balancing a target load due to fluctuations of air charging efficiency. This is because air charging efficiency is closely related to engine output torque. For example, because the direct injection-spark ignition type of engine is designed and adapted to make stratified charge combustion by performing fuel injection in compression strokes while the engine operates with lower engine loads and, during the stratified charge combustion, the target air-fuel ratio is varied so as to provide a lean air-fuel mixture, the engine output torque drops if simply controlling the amount of intake air linearly corresponding to a change in accelerator pedal travel. In order to avoid such a drip in engine output torque, it has necessary to increase the amount of intake air while the air-fuel mixture remains lean. For this reason, it has been proposed to control the amount of intake air on the basis of a target engine load, i.e. to control throttle opening according to a target engine load, as well as the target air-to-fuel ratio. However, because, during a transitional engine operating state in which the target engine load varies due to treading on an accelerator pedal, a change in the amount of intake air occurs after a delay from the change in target engine load, it is not impossible to gain output torque balancing the target engine load even when controlling the amount of fuel injection so as to balance the target air-fuel ratio determined on the basis of the target engine load while the air charging efficiency differs from a value corresponding to the target engine load due to the delay.